New vehicular power transmission technologies, such as infinitely variable transmissions (IVTs) and traction drives, are currently being developed based upon torque transmission between smooth rolling elements. These new transmissions require lubrication between the rolling elements, and torque is actually transmitted through the lubricant film. Therefore, the shear strength properties of the lubricant under the high shear stress conditions of the contacts govern the level of power throughput capability and efficiency. Thus lubricants with high elastohydrodynamic shear strength are required for these applications but equally, as in normal geared transmissions, it is preferable for the lubricants to exhibit low dependence of viscosity on temperature (high Viscosity Index) and good low temperature viscometrics.
However, fluids that have high elastohydrodynamic shear strength have generally been found to have poor temperature-viscosity behavior and poor low temperature viscometric properties. This is due to the fact that the structural features of fluids that impart high shear strength generally also impart poor temperature-viscosity behavior, both in terms of low temperature dynamic viscosity and Viscosity Index (VI). Thus, few lubricants are currently marketed as being suitable for use in IVT's and these are not only very expensive but they tend to have such poor temperature-viscosity properties, especially at low temperatures, as to be considered inadequate for IVT fluid application in many parts of the world, including the United States of America.
There is therefore a need for fluids that combine good high pressure shear strength properties with good temperature-viscosity properties.
According to the invention, it has been found that combining certain diphenyl-, phenylcyclohexyl- and bis-cyclohexyl-alkane fluids, mixed fluids are obtained that have high elastohydrodynamic shear strength and improved temperature-viscosity behavior.
It is known from, for example, U.S. Pat. No. 3,994,816 that power in a hydraulic transmission system can be transmitted by a synthetic fluid comprising substantially completely hydrogenated dimers of alpha-alkyl styrenes, such as 2,4-dicyclohexyl-2-methylpentane. In particular, the synthetic fluid composition disclosed in the '816 patent comprises a major amount of hydrogenated dimers of α-methylstyrene having less than about 2% unsaturation, the fully hydrogenated dimer consisting essentially of at least about 80% by weight of fully hydrogenated dimer of a linear dimer and from 0% to 20% by weight of fully hydrogenated dimer of a cyclic dimer. However, although 2,4-dicyclohexyl-2-methylpentane exhibits high elastohydrodynamic shear strength, its temperature-viscosity properties are very poor.
Japanese Kokai Patent Application No. 09-227888A discloses a fluid for a traction drive having increased oxidation resistance and comprising (a) 0.5 to 20 weight %: of one or more of the following hydrocarbons: 2-methyl-2,4-diphenylpentane; 2-methyl-4-cyclohexyl-2-phenylpentane; 2-methyl-2-cyclohexyl-4-phenylpentane; 2,4-dimethyl-2,4,6-triphenylheptane; 2,4-dimethyl-2-cyclohexyl-4,6-diphenylheptane; 2,4-dimethyl-4-cyclohexyl-2,6-diphenylheptane; 2,4-dimethyl-6-cyclohexyl-2,4-diphenylheptane; 2,4-dimethyl-4,6-dicyclohexyl-2-phenylheptane; 2,4-dimethyl-2,6-dicyclohexyl-4-phenylheptane; and 2,4-dimethyl-2,4-dicyclohexyl-6-phenylheptane, with the balance consisting of a base material of a fluid commonly used for traction drive, including, for example, hydrides of dimers or trimers of α-methylstyrene, such as 2-methy-2,4-dicyclohexylpentane; 2-methyl-2,4-dicyclohexylbutane; 2,4-dimethyl-2,4,6-tricyclohexylheptane; hydrides of indan compounds formed from dimers of α-methylstyrene, hydrides of dimers through hexamers of cyclopentadiene that may also be alkylated, and alkylated decalin compounds, such as trimethyl decalin and isopropyl decalin. The hydrocarbons (a) may be produced by the partial hydrogenation of the dimers and trimers of α-methylstyrene.